Support configuration of OpenMP in backend for HiOp

[cameronrutherford]

#562 is a bandaid fix to our OpenMP support.

In order to support specification with Spack/CMake the following needs to happen:

• Add OpenMP option to CMake that controls when OpenMP is included and linked
• Set RAJA execution policy to sequential when using RAJA and no other backend is enabled (OpenMP, CUDA, HIP, etc)
• Add exported CMake configuration so that ExaGO can use HiOp's OpenMP
• Add testing with and without OpenMP (unless xSDK testing on MacOS is serving this purpose)
• Add CMake logic to warn about OpenMP in HiOp when building with Apple Clang
• In the spack package, make sure OpenMP is disabled when Apple Clang is being used

https://cmake.org/cmake/help/v3.25/variable/CMAKE_LANG_COMPILER_ID.html#variable:CMAKE_%3CLANG%3E_COMPILER_ID - while it is possible to include logic do detect when Apple Clang is being used, we will only warn about it at the CMake level. Instead prefer logic to reside in Spack, as you can work around OpenMP with Apple-Clang issues by installing OpenMP with brew for example.

This should probably be done before a more serious refactor to address #240 and #241

cc @pelesh @nychiang @cnpetra

[cnpetra]

#240 and #241 are addressed by PR#543. I wanted to schedule meeting to get in sync with you, but didn't have the chance. Expect one the next couple of weeks.

[pelesh]

Set RAJA execution policy to sequential when using RAJA and no GPU backend

Shouldn't this read "... when using RAJA and no GPU or OpenMP CPU backend"?

[cameronrutherford]

Set RAJA execution policy to sequential when using RAJA and no GPU backend

Shouldn't this read "... when using RAJA and no GPU or OpenMP CPU backed"?

Agreed. I updated the description